Ball grid array (BGA) packages, such as multi-chip modules (MCMs), are small circuit boards that combine several chips, usually a microprocessor and support chips, into one convenient subsystem. BGA packaging is ubiquitous aspect of modern electronic miniaturization and micro-electronic systems. BGA packages use a ball grid array (BGA) to make electrical connection with a printed circuit board. The BGA is an improvement over the pin grid array, which is a package with one face covered with pins in a grid pattern. The pins connect the integrated circuit to a printed circuit board (PCB) on which it is placed.
The BGA is a solution to the problem of producing a miniature package for an integrated circuit with many hundreds of pins. Pin grid arrays and dual-in-line surface mount packages are being produced with more and more pins, and with decreasing spacing between the pins. As package pins get closer together, the danger of accidentally bridging adjacent pins with solder increases. BGAs solve this problem.
FIG. 1 illustrates a typical BGA package 100. As can be seen in FIG. 1, the pins are replaced by a ball grid array 105 formed of balls of solder 110 on the bottom 115 of the BGA package 100. The bottom 115 of the BGA package 100 is the bottom surface of a device, such as, but not limited to a MCM.
The device is assembled on a PCB with copper pads (not shown) in a pattern that matches the solder balls 110. The assembly is heated, melting the solder balls 110 to the PCB. Surface tension holds the molten solder and the package in alignment with the circuit board while the solder cools and solidifies.
There are several reasons for directly probing a ball grid array package prior to melting the BGA solder balls. Existing sockets for BGAs, often needed during development, burn-in, rework, inspection, troubleshooting, and testing tend to be unreliable. There are two common types of socket. The more reliable type has spring pins, commonly referred to as “pogopins”, that push up under the balls. The less reliable type is a ZIF socket, with spring pinchers that grab the balls. Neither of these systems work well, without sufficient uniform downward force applied to the MCM package or integrated circuit, especially if the balls are small.
FIG. 2 illustrates a typical test socket assembly 200. The test socket assembly 200 includes a test pad 210, an interlock portion 215, electrical connections 216 that lead from the test pad 210 to an analysis device (not shown), and a push plate 220. The test pad 210 includes a plurality of pins 212 for receiving a ball grid array of a device to be tested. The test pad 210 may include a removable package holder (not shown, see FIG. 5), that positions the device or package to be tested upon test pad 210. The interlock portion 215 is where the push plate 220 is attached to the test socket assembly 200. The interlock portion 215 includes protrusions 217 for engaging the push plate 220.
The push plate 220 includes openings 221 for receiving and engaging the protrusions 217. The openings 221 are configured so that when the protrusions 217 are received in the openings 221 and the push plate 220 is rotated, the push plate 220 becomes engaged and/or locked into position with the interlock portion 215. The openings 222 of the push plate 220 may be configured to pull the push plate 220 towards the test pad portion 210 when the push plate 220 is rotated, thus applying a force to the device to push or apply force to the device towards the test pad portion 210 thereby assuring contact between the ball grid array of the device and the pins of the test pad 210.
To test a device, such as a MCM, the device is placed upon test pad 210 with the BGA ball side of the device in contact with the pins of the test pad 210. The push plate 220 is placed upon the device with the protrusions 216 of the interlock portion 215 received in openings 222 of the push plate 220. The push plate 220 is rotated to lock the device into position against the test pad 210.
In other designs, the push plate may be attached to an interlock portion by other attachment methods and devices, such as screws and/or clamps. In still other designs, a push plate may be initially attached to the interlock portion and opened to receive the device to be tested by a clam shell or lever action.
FIGS. 3A and 3B illustrate other typical push plate designs. FIG. 3A illustrates a first push plate 310 that is attached to an interlock portion by screws 312. Knob 314 may be then used to push the test device 316 against the test pad 310. FIG. 3B illustrates a second push plate 350 attached to an interlock portion 352 by screws 354. This type of push plate design may be referred to as a clam shell design. In this design, the second push plate 350 may be opened to receive a device to be tested (not shown) upon the test pad portion 356. The top portion 360 of the second push plate 350 may then be closed to secure the BGA of the device against the pins 362.
These push plates illustrate the root of the problem in using these typical push plates, in that these existing methods to clamp the ball grid array package to the temporary socket are clumsy and block access to the ball grid array package. In some existing push plate designs, there may be a small opening on that socket, but it is for the operator to determine if a chip is installed in the socket without opening the lid.
As discussed above, the ball grid array package must be held against the spring pins, called POGO pins, with the push plate with enough force to make good contact but not with enough force to damage the solder balls or the PCB itself. This holding force must be applied uniformly to the ball grid array package, without large torques, since torques can cause excessive force along an edge or at a corner of the solder ball array, causing damage. Thus, the top holder for the test socket must supply a known, even, down force, without excessive torques. Current push plate designs, for instance the clam-shell design as shown in FIG. 3B fail to apply an even down force.
In addition to these force and torque requirements, there are many instances where physical access to the top of some portion of the ball grid array package chips is needed, for example for failure analysis applications, and this is universally not possible with current sockets, as pointed out previously. Current push plate methods do not allow this access because of the methods used to supply the down force.
Furthermore, push plate assemblies may be used as an installation tool for mounting a ball grid array package onto an application PCB. This application has the same problems associated with the inspection and temporary socket application described above, limited visual access to the ball grid array package and uneven force applied to the mounted ball grid array package.
The need remains, therefore, for a device and method that can attach a ball grid array package, such as a MCM, to a circuit board or test socket that allows for direct probing of the package and that applies an even down force to the package while enabling unfettered access to the top side of the MCM package.